High molecular weight linear phosphonitrilic chloride polymers are used to make a broad range of useful polyphosphazenes by substitution of the chlorine atoms with various groups such as phenoxide, alkylphenoxide, fluoroalkoxide and the like. These polyphosphazenes can be used to make articles such as foam insulation, old-rings, hydraulic seals, wire and cable insulation, hose liners and the like. The high molecular weight linear phosphonitrilic chloride polymers used to make the various substituted polyphosphazenes can be made by the thermal polymerization of cyclic phosphonitrilic chloride trimer preferably using a catalyst as described in U.S. Pat. Nos. 4,123,503 and 4,226,840. Not all trimer can be successfully polymerized to form a linear polyphosphonitrilic chloride. Early attempts at the thermal polymerization of phosphonitrilic chloride trimer lead to the formation of hard, cross-linked polymers that were of little use. The secret to obtaining linear polymers rather than the hard, cross-linked polymers was found to be related to the purity of the phosphonitrilic chloride trimer. Various methods have been tried to purify trimer to obtain trimer that could be successfully polymerized to high molecular weight linear polymers. Such purified trimers are referred to herein as "polymerization grade trimers". Various purification methods are described in U.S. Pat. Nos. 3,008,799; 3,378,353; 3,379,510; 3,677,720; 3,694,171; 3,952,086; 4,175,113 and GB Pat. No. 2,114,111.